At present, there are two approaches for testing the integrity and quality of electrical interconnections upon PWBs. A first approach is based upon a Bed of Nails (BON) tester. A BON tester employs a large number of test pins that are simultaneously contacted to the PWB under test to provide a rapid test that is well suited to high volume manufacturing environments. However, BON testers rely upon product-specific, custom-built test fixtures that require a significant investment of both development time and money. Also, as circuit feature sizes become smaller, BON fixturing and testing becomes increasingly difficult. At present, a lower limit of circuit feature spacing than can be reliably tested by the BON approach is approximately 0.025 inches. Circuit feature spacing below 0.025 inches is sometimes accommodated by the BON approach, for small portions of the product under test, but at an additional expense. Furthermore, as circuit fabrication technology evolves, the gap between substrate test requirements and BON capabilities will undoubtedly grow wider.
In response to the shortcomings of BON testers, flying probe testers have been developed. As depicted in FIG. 1 a flying probe tester employs a motion system to move at least one test probe 1 to circuit nodes 2 on a PWB 3 under test. As indicated by the arrow (A) a test cycle consists of raising the probe 1 off of the PWB, moving the probe 1 to a position over a next circuit node to be tested, moving the probe 1 down to contact the node 2, and making an electrical measurement.
One advantage of flying probe testers is that no custom-built, product-specific fixtures are required. A control program that drives the probe 1 is typically automatically derived from a PWB CAD data base. Another advantage of flying probe testers is that circuit feature spacing as small as 0.004 inches and less can be reliably tested.
However, a primary disadvantage of the flying probe tester is the significant amount of time required to test a complex PWB. During each measurement cycle the flying probe tester spends a significant amount of time waiting for the motion system to move, stop, and settle. In that there are three move-stop-settle delays during each measurement cycle, flying probe testers may require tens of minutes to test a PWB that require only a few minutes on a BON tester.
In U.S. Pat. No. 4,565,966, issued Jan. 21, 1986, entitled "Method and Apparatus for Testing of Electrical Interconnection Networks" to Burr et al. there is described a test system of the flying probe type. A first probe is lowered into contact with the network and the capacitance of the network is measured with respect a reference plate. The capacitance measurement detects open circuit and short circuit conditions of the network. Next, a second probe is lowered into contact with another portion of the network and a resistance measurement is made between the two probes. Capacitance values for a network may be calculated, although a preferred approach is said to learn the capacitive values by direct measurements from a PWB without defects.
The approach taken by Burr et al. requires that the probes be lifted from the PWB, translated to a new position, and lowered back to contact a network being tested. As such, this system of the prior art suffers from the aforementioned disadvantage inherent in the flying probe approach to PWB testing.
It is therefore one object of the invention to provide an electrical network test method and apparatus that overcomes the problems of the prior art by accomplishing testing of a plurality of nodes during a continuous motion of a test probe.
It is a further object of the invention to provide a PWB tester that consecutively measures a capacitance of each of a plurality of adjacent nodes during a time that a test probe is in continuous motion relative to the PWB.
It is another object of the invention to provide a PWB tester that consecutively measures a charge capacity of each of a plurality of adjacent nodes during a time that a test probe is in continuous motion relative to the PWB.
It is a further object of the invention to provide a PWB tester that measures capacitance, termination resistance, and in-circuit characteristics with a continuous motion contact or non-contact probe.